Scientists are investigating a mysterious kind of pneumonia. They call it Severe Acute Respiratory Syndrome, or SARS. Most of the cases have been among health care workers in Hong Kong, Hanoi and Singapore. These workers were directly involved in the care of people infected with the disease. Family members of those infected have also gotten sick. Several people have died.

Some people have been covering their nose and mouth with masks to try to avoid infection. But the World Health Organization -- the W-H-O -- says the disease does not spread easily. Experts say it requires close contact. They say the sickness may be passed in body fluids, including the fluid released when a person coughs or sneezes.

The W-H-O declared SARS a worldwide health threat on Saturday. But, as of Wednesday, the United Nations agency said there is no need to restrict travel to any country.

Still, health officials are concerned that people who travel on airplanes could spread Severe Acute Respiratory Syndrome to more countries. Some airports in Asia and the Pacific have been testing passengers for signs of the disease.

The main signs of the disease are a body temperature of more than thirty-eight degrees Celsius, a cough, and breathing difficulties.

On Wednesday the W-H-O said officials were investigating reported cases in Canada, mainland China, Taiwan, Germany, Hong Kong and Singapore. There were also reports in Slovenia, Thailand, Vietnam and the United Kingdom.

Some experts say they believe the sickness is a new form of influenza. It appears to resist traditional antibiotic medicines. The W-H-O has brought together laboratories in ten countries to identify the cause and develop a treatment.

The agency reports that SARS was first recognized in Hanoi on February twenty-sixth. A hospital in the Vietnamese capital admitted an American businessman. It was not known where he had become infected. The man was later moved to a hospital in Hong Kong where he died. Health workers at both hospitals also became sick. One died.

The W-H-O says Chinese officials have provided a report on more than three-hundred cases of what may be the same or a related disease. The pneumonia began in Guangdong province, in southern China, in November. It is now said to be under control.